Combine 2 normal maps (1 for layout and 1 for detail)
You can mix the 2 textures into a MixRGB node. Put the MixRGB factor at 0.5, and put the Normal Map strength at 2. As they are not supposed to have the same scale, they both use the same UV map but the first one will use a Mapping scale of 1 while you'll use a bigger scale for the second one (here x 10):